Construction of PBGA substrate for flip chip packing

ABSTRACT

The present invention discloses techniques that improve the reliability of a flip packages that uses underfill encapsulation. One embodiment of the present invention describes a method and apparatus of packaging a flip chip by relocating the neutral plane of the package substrate away from its mid-plane. Another embodiment of the present invention describes a method and apparatus of arranging the layers of a laminate for use in PBGA packaging that arranges the layers of the laminate according to the stiffness of each layer. Another embodiment of the present invention describes a method and apparatus of packaging a flip chip that uses one or more redundant interconnections at the bottom of the package substrate where the redundant interconnections are within the shadow of the IC chip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to packaging semiconductor devices. Morespecifically, this invention relates to packaging semiconductor deviceswith laminar substrates using the flip chip packaging technique.

2. Description of the Related Art

The drive to higher semiconductor device densities requires that thepackaging of a device or chip support these new densities. One techniquethat supports the increased device densities is the shift fromperipheral wire bonding to area array chip interconnects. Area arraychip interconnects use bumps or solder joints that directly couples thesemiconductor chip to the package substrate. This technique accommodatesthe increased I/O pad counts and brings current to the interior of thechip, which improves the voltage noise margins. One type of area arrayinterconnect packaging technique is the flip chip (FC) solderinterconnect on a substrate. In the flip chip assembly or package, thesemiconductor device or integrated circuit (IC) chip typically has bumpsfor soldering (FC solder joints) manufactured on the I/O pads of thechip (the top or etched side of the IC chip), which takes the place ofthe wire bonding pads. The chip is then turned upside down or "flipped"so that the device side or face of the IC chip couples to a substratesuch as found in a plastic ball-grid-array (PBGA) substrate (orlaminate).

One problem associated with the flip chip packaging technique occursbecause of the large differences or mismatch between the coefficient ofthermal expansion (CTE) of silicon (the IC chip) and the CTE of the PBGAsubstrate. The CTE of silicon is about 3 ppm per degree Centigrade andthat of the PBGA substrate is about 17 ppm per degree Centigrade. Thelarge difference in CTE causes the solder joints between the IC chip andthe PBGA substrate to undergo shear stress during temperature changes,which can occur during the regular manufacturing process or even duringthe daily use of the package when used in a product. This repetitiveheating up and cooling down of the package can cause reliabilityproblems in a flip chip package that may cause it to fail prematurelyand or suffer degraded performance.

One technique used to transfer some of the shear stress away from the FCsolder joints is to use an encapsulant as an underfill material betweenthe device side of the IC chip and the top surface of the PBGAsubstrate. After the encapsulant cures, the bonding of the encapsulantshifts most of the shear stress away from the solder joints andredistributes the stress across the IC chip, the substrate, and theencapsulant as a whole (also known as the package, the assembly, or theencapsulated flip chip package). Although the use of an encapsulantreduces some problems, its use creates new problems. One problem createdwhen using an encapsulant is warpage. Warpage occurs because of themismatches of the CTE of the IC chip and the CTE of the PBGA laminate.When the encapsulant cures, the substrate and the chip are initiallyflat. After the encapsulated FC PBGA cools down (to room temperature),the substrate will shrink more than the IC chip and cause the back sideof the IC chip to be higher in the center (bulge up) and lower at thecorners. This warpage in the assembled flip chip package causescompression stresses on the device side of the IC chip and tensilestresses on the back side of the IC chip. An excessive tensile stressmay cause the IC chip to crack, and an excessive stress on the flip chippackage may cause interfacial delamination between the IC chip and theencapsulant and or between the encapsulant and the substrate.

The present invention is a collection of techniques that overcome theabove disadvantages and improve the overall reliability of the flip chippackaging technique. One may practice the different techniques of thepresent invention separately or in combination with one another. Thepresent invention will reduce the probability of an IC chip crack due toexcessive stresses (this also includes reducing the probability ofstress enhanced electromigration of the metal lines on an IC chip) thatincludes warpage. Additionally, the present invention will reduce theprobability of an interfacial delamination between the IC chip and theencapsulant and or between the encapsulant and the substrate. And, thepresent invention will also improve the reliability of the flip chippackaging technique in its daily use under repetitive thermal cycles ofnormal use when used as part of a product.

The present invention discloses one embodiment that relocates theneutral plane in a PBGA laminate. This embodiment of the presentinvention reduces the compression stress on the device side of the ICchip and reduces the tensile stress on the back side of the IC chip thatoccurs when the encapsulated flip chip package cools from its wiringboard assembly temperature to room temperature.

Another embodiment of the present invention controls the stiffness ofindividual layers within the PBGA laminate with a gradual reduction ofstiffness from the bottom side to the top side of the laminate. Thisembodiment reduces warpage and the shear stress on the IC chip solderjoints.

Another embodiment of the present invention uses redundant solder jointconnections between the encapsulated flip chip package and the printedwiring board. These additional solder joint connections help reduce thebending of the IC chip and help improve the placement of any heat sinkon the back side of the IC chip. Reducing the bending of the IC chipalso helps improve its thermal performance with a heatsink. The thermalperformance is especially important when using high power IC chips thatrequire a high capacity heat sink.

SUMMARY OF THE INVENTION

The present invention discloses a collection of techniques that improvethe reliability of a flip chip (FC) plastic ball grid array (PBGA)assembly. One may practice the different techniques of the presentinvention separately or in combination with one another. The presentinvention reduces cracking of the IC chip due to excessive tresses suchas warpage. Additionally, the present invention reduces the interfacialdelamination between the IC chip and the encapsulant and or between theencapsulant and the substrate. And, the present invention improves thereliability of the flip chip packaging technique under repetitivethermal cycles of normal use.

One embodiment of the present invention describes a method and apparatusof packaging a flip chip by relocating the neutral plane of the PBGAsubstrate away from its mid-plane. One technique to relocate the neutralplane is to relocate the neutral plane during the fabrication of thestructure.

Another embodiment of the present invention describes a method andapparatus of arranging the layers of a laminate for use in flip chippackaging that arranges the layers of the laminate according to thestiffness of each layer. The present invention arranges the layers inthe laminate from the top to the bottom of the laminate where the leaststiff layer is at the top of the laminate and the most stiff layer is atthe bottom of the laminate. One technique to vary the stiffness of themetal layers is to vary the diameter of the via holes that go throughthe metal layer.

Another embodiment of the present invention describes a method andapparatus of packaging a flip chip assembly that uses one or moreredundant interconnections between the PBGA substrate and the printedwiring board. The redundant interconnections are at the bottom of thepackage substrate and are within the shadow of the IC chip. The presentinvention routes all signal paths and some of the power and groundconnections to areas on the bottom of the package substrate that areoutside the shadow of the IC chip with non-critical redundant power andground connections routed to the area within the shadow of the IC chip.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flip chip packaging technique.

FIG. 2 illustrates common reliability problems associated with the flipchip packaging technique.

FIG. 3 illustrates the current practice for locating the neutral planeswithin a flip chip package.

FIG. 4 illustrates a geometric technique for approximating the effectiveCTE at the neutral plane of the encapsulant within a flip chip package.

FIG. 5 illustrates an embodiment of the present invention that relocatesthe neutral plane of a substrate.

FIG. 6 is a cross section of an example substrate in current practice.

FIG. 7 illustrates an embodiment of the present invention forcontrolling the stiffness of a substrate.

FIGS. 8A and 8B illustrate the current practice of locating solder ballson the bottom of a substrate.

FIGS. 9A and 9B illustrate an embodiment of the present invention thatuses additional solder balls on the bottom of a substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses methods and apparatuses that improve thereliability of a flip chip plastic ball grid array package that usesunderfill encapsulation to help couple an integrated circuit chip to theplastic ball grid array laminate. This disclosure describes numerousspecific details that includes specific materials, structures, andprocesses in order to provide a thorough understanding of the presentinvention. For example, this disclosure describes the present inventionin terms of a flip chip packaging technique. One skilled in the art,however, will appreciate that one may practice the present inventionwithout these specific details. And finally, this disclosure does notdescribe some well known processes, specific materials, andsemiconductor structures in detail in order not to obscure the presentinvention.

A brief review of the current practice of flip chip (FC) assembly withplastic ball grid array packaging will help illustrate the advantages ofthe present invention. FIG. 1 illustrates a typical flip chip PBGAassembly 9 that comprises a semiconductor device or an integratedcircuit (IC) chip 10 coupled to a PBGA substrate or laminate 20 throughthe IC solder joints or bumps 18. An encapsulant 16 couples the IC chip10 to the laminate 20. The flip chip process "flips" the IC chip upsidedown so that the device side or face side of the IC chip is the sidecoupled to the PBGA laminate. For purposes of this disclosure,therefore, the bottom of the IC chip 12 is the device side of the ICchip and the top of the IC chip 14 is back side of the IC chip. The flipchip typically has bumps for soldering (IC solder joints) 18manufactured on the I/O pads of the IC device. The IC solder joints takethe place of the bonding wires used in non-flip chip packaging.

The PBGA laminate 20 couples to the printed wiring board (PWB) 11through the PBGA solder joints 22. The plated through holes (PTH) 13connect the signals from the IC chip 10 through the laminate 20 on toPWB 11. The electrical signal path is from IC chip 10 through IC solderjoints 18 through substrate 20 by way of the PTHs 13. From the PTHs, thesignals go through the PBGA solder joints 22 to PWB 11.

FIG. 2 illustrates common reliability problems associated with the flipchip packaging technique. Delamination 100 is a separation of layers andit can occur at various interfaces including the interface between theIC chip 10 and IC solder joints 18, the IC solder joints 18 and thesubstrate 20, the substrate 20 and the PBGA solder joints 22, and thePBGA solder joints 22 and the PWB 11. Warpage 102 is an out of planedisplacement and is a manifestation of the mismatches of the coefficientof thermal expansion (CTE) of the IC chip and the CTE of the PBGAlaminate. After the encapsulant 16 cures and subsequently cools down toroom temperature, the PBGA laminate 20 shrinks more than the IC chip 10and causes the back side 14 of the IC chip to be higher in the center(bulge up) and lower at the corners. Since silicon is basically abrittle material, chip cracking 104 may occur because of excessivestresses placed on the IC chip 10 such as from warpage. The warpage inthe assembled flip chip package causes compression stress on the deviceside 12 of the IC chip 10 and tensile stress on the back side 14 of theIC chip. And, warpage can also cause solder strain 106 to occur becauseof the tensile or compression forces placed on the solder joints such asthe IC joints 18.

Relocating the Neutral Planes

FIG. 3 illustrates the current practice for locating the neutral planeswithin a flip chip package assembly. When bonding an IC chip 10 to aPBGA substrate 20 with an encapsulant 16, a flip chip package assembly 9(IC chip, encapsulant, and PBGA substrate) will bend or warp toward thehigh CTE side. Warpage (102 of FIG. 2), as previously discussed, occursbecause of the mismatches of the CTE of the IC chip 10 and the CTE ofthe PBGA laminate 20. After the encapsulant 16 cures and subsequentlycools down to room temperature, the substrate 20 shrinks more than theIC chip 10 and causes the back side of the IC chip 10 to be higher inthe center (bulge up) and lower at the corners. The warpage in theassembled flip chip package causes compression stress on the device side12 and tensile stress on the back side 14 of the IC chip 10. Anexcessive tensile stress may cause the IC chip to crack, and anexcessive stress on the flip chip package assembly 9 may causeinterfacial delamination between the IC chip 10 and the encapsulant 16and or between the encapsulant 16 and the substrate 20.

By itself, a typical IC chip 10 has a neutral plane 24 at the middle ofthe chip thickness (its mid-plane). A neutral plane is the plane withina structure where the stress is neutral, while the mid-plane of astructure is the plane that is half way between the top and the bottomof the structure. Additionally, a typical PBGA laminate or substrate 20has a neutral plane 26 at the middle of the laminate (or its mid-plane).During the cooling down of the flip chip package assembly 9 afterencapsulation, we may approximate that the neutral plane 24 of the ICchip 10 will shrink at a coefficient of thermal expansion (CTE) of 3 ppmper degree Centigrade, and the neutral plane 26 of the laminate 20 willshrink at a CTE of 17 ppm per degree Centigrade. And, the neutral plane28 of the encapsulant 16 will shrink at an "equivalent" CTE that issomewhere between 3 and 17 ppm per degree Centigrade.

It is possible to calculate or simulate with some degree of precisionthe degree of bending of the IC chip 10 and the PBGA substrate 20 andthe amount of shift from shear stress to bending stress by knowing themodulus and CTE of the IC chip, the cured encapsulant, and the PBGAlaminate. These calculations are cumbersome and computationallyintensive. However, one can approximate this analysis by using ageometric analysis to estimate the above stresses.

FIG. 4 illustrates a geometric estimation for approximating theeffective CTE at the neutral plane of the encapsulant within a flip chippackage assembly. As part of this estimation, one needs to assume thatthe modulus of the encapsulant is much smaller than those of the IC chip10 and the PBGA laminate 20, and that the tensile and shear moduli ofthe IC chip and the laminate substrate are the same.

From above, one knows that the CTE of the laminate 20 (and it's neutralplane 26) is 17 ppm per degree Centigrade, and CTE of the IC chip 10(and it's neutral plane 24) is 3 ppm per degree Centigrade. The linesegment DE corresponds to the CTE of the IC chip's neutral plane 24. Theline segment BC corresponds to the laminate's neutral plane 26. And, themid-plane 28 of the encapsulant 16 corresponds to the line segment XY.One can calculate the "equivalent" CTE of the mid-plane 28 of theencapsulant 16 by knowing the distance from the neutral plane of theencapsulant (which here is its mid-plane) to the neutral planes of thechip and the PBGA laminate. If the distances are equal, the "equivalent"CTE will be 10 ppm per degree Centigrade.

For example, if the distance between the laminate'neutral plane 26 andthe mid-plane 28 of the encapsulant is 14×10⁻³ inches or 14 mils (linesegment BX) and the distance between the neutral plane 24 of the siliconIC chip is also 14 mils (line segment DX), from geometry thisrelationship yields:

    XY (CTE encapsulant mid-plane)=(BC*DX+DE*BX)/(BX+DX),

or

    XY=(17×14+3×14)/(14+14)=10 ppm per degree Centigrade

This shows that the approximate CTE of the encapsulant is 10 ppm perdegree Centigrade. As defined earlier, warpage occurs because of themismatch of the CTE of the laminate to the CTE of the IC chip. However,if one positions the location of the laminate's neutral plane 26 awayfrom the encapsulant 16, one can reduce the CTE of the neutral plane 28of the encapsulant.

FIG. 5 illustrates an embodiment of the present invention that relocatesthe neutral plane of a substrate. Relocating the neutral plane of thesubstrate will reduce the compression strain on the device side of theIC chip and reduce the tensile strain on the back side of the IC chipthat occur when the encapsulated flip chip package cools from its wiringboard assembly temperature to room temperature. The following exampleillustrates the reduced stresses produced by the present invention: ifthe distance of the IC chip's neutral plane 24 (DE of FIG. 4) is 8 milsfrom the mid-plane 28 of the encapsulant and the laminate's neutralplane 26 (BC of FIG. 4) is 20 mils from the mid-plane 28 of theencapsulant, using the geometrical model illustrated in FIG. 4 yields:

    XY=(17×8+3×20)/(8+20)=7 ppm per degree Centigrade

Therefore, the "equivalent" CTE at the neutral plane 28 of theencapsulant is now 7 ppm per degree Centigrade.

At the device side of bottom 12 of the IC chip 10, the "equivalent" CTEat the interface between the IC chip 10 and the encapsulant 16 will bebetween 3 and 7, and probably around 6.5 ppm per degree Centigrade. This"equivalent" is 3.5 ppm greater than the 3 ppm for the IC chip, insteadof 6 ppm greater when the two neutral planes were at an equal distancefrom the mid-plane 28 of the encapsulant. This represents a 42%reduction in the compression strain on the device side of the IC chip.

The preferred embodiment of the present invention for relocating theneutral planes has the location of the neutral plane of the substratemoved during the fabrication of the laminate. One such technique forrelocating the neutral plane is by controlling the stiffness of the PBGAsubstrate, as described below.

Controlling the Stiffness of the PBGA Substrate

Another embodiment of the present invention that improves thereliability of a flip chip assembly is the control or alteration of thestiffness of the individual layers within a PBGA laminate. The typicalpractice in manufacturing the separate layers of a PBGA substrate(including both metal and dielectric layers) is to make each layer withthe same relative stiffness as the other similar layers. That is eachdielectric layer has the same relative stiffness as the other dielectriclayers, and each metal layer has the same relative stiffness as theother metal layers. The glass transition temperature and the Young'smodulus of these layers may be different from each other, but thecoefficient of thermal expansion (CTE) for each layer needs to berelatively the same as each other layer, especially for the metal layersin the laminate.

Referring to FIG. 6, an example PBGA substrate or laminate 49illustrates the current practice for manufacturing the laminate, whichcomprises the dielectric layers 40, 42, and 44 and interspersed betweenthe dielectric layers are the metal layers 46, 48, 50, and 52. The metallayers further comprise a signal layer 46, a voltage layer 48, a groundlayer 50, and a signal layer 52. The dielectric layers 40, 42, and 44all have the same relative stiffness to each other, the metal signallayers 46 and 52 have the same relative stiffness to each other (theselayers typically comprise a surface area of 10% to 30% metal such ascopper), and the metal voltage and ground layers 48 and 50 have the samerelative stiffness to each other (these layers typically comprise asurface area of 75% to 85% metal). The means that the overall stiffnessof the substrate 49 is basically symmetrical from top to bottom.

The typical laminate 49 further comprises the plated through holes (PTH)54, 56, and 58 for conducting signals from an IC chip through thelaminate 49 to a printed wiring board (see for example FIG. 1). Anindividual PTH may be formed by a mechanical drilling process or by alaser drilling process for example. After formation of the hole for thePTH, the hole is plated with a metal layer 55 for conducting thesignals. PTH 54 is an example of a plated through hole coupled to theground layer 50. PTH 56 is an example of a plated through hole coupledto the voltage layer 48. And, PTH 58 is an example of a plated throughhole conducting a signal from the signal layer 46 to the signal layer52.

When a PTH connects to a layer in the laminate, the metal plated layerof the PTH contacts the metal layer. For example in PTH 56, its metalplated layer 57 couples to the voltage layer 48 at point 64. The outerdiameter 62 of PTH 56 (including the metal plated layer) is about 18mils (thousandths of an inch) (where the inner diameter of the interiorof PTH is typically 12 mils). When a PTH does not connect to a metallayer such as PTH 54 and metal layer 48, the diameter of the clearancehole 61 through the metal layer 48 is about 20 mils, which leaves a gap60 of about 4 mils between the metal layer 48 and the metal plated layer55 of PTH 54.

This embodiment of the present invention improves the reliability offlip chip package by controlling or varying the stiffness of the PBGAlaminate by arranging the metal layers of the laminate accordingly sothat the stiffest layer is towards the bottom of the laminate and theleast stiff layer is towards the top of the laminate. The preferredembodiment of the present invention varies the stiffness of the voltageand ground layers (or the inner layers). For example with reference toFIG. 7, the relative stiffness of the layers would increase from theupper layer of the laminate comprising a metal layer 48 being stiff anda metal layer 50 being stiffer. Additionally, the top layer of thelaminate (the layer that contacts the encapsulant), which could be asolder mask, should have a glass transition temperature near that of theencapsulant.

Controlling or varying the stiffness of the laminate is one technique torelocate the neutral plane of the laminate away from its mid-plane. Aspreviously discussed, relocating the laminate's neutral plane away fromthe encapsulant allows one to reduce the CTE of the neutral plane of theencapsulant.

Referring now to FIG. 7, one can control the stiffness of the metallayers, which typically comprise copper as the metal, by altering thediameter of the clearance holes within the layer. For example,increasing the diameter 80 of the clearance hole around PTH 54 to 30mils on metal layer 48 reduces the stiffness of the layer. The reductionof stiffness occurs because there is less metal in the layer 48 (overallwhen compared to the prior method of making the substrate as in FIG. 6)and larger clearance holes around the PTHs. Together, thesemodifications combine to reduce the stiffness of a metal layer (or givethe metal layer greater flexibility). In contrast to the largerclearance hole, the diameter 82 of the clearance hole around PTH 58 isabout 18 mils (similar to the size of the prior method of making theclearance holes as illustrated by FIG. 6). By not changing the size ofthe clearance hole, the stiffness of layer 50 has not changed, althoughit is now greater than the stiffness of layer 48 due to the changes inlayer 48. By reducing the stiffness of metal layer 48, it is now lessstiff than metal layer 50. In other words, the order of relativestiffness of the metal layers is that metal layer 48 is stiff and metallayer 50 is stiffer.

Sacrificial Solder Balls

Another embodiment of the present invention that improves thereliability of a flip chip assembly helps reduce the stresses on thesolder joint connections between a PBGA laminate and a PWB (PrintedWiring Board). Additionally, this embodiment helps reduce the bending orwarpage of the IC chip. Bending or warpage of the IC chip may preventthe use or placement of a heatsink coupled to the top side of IC chip.Thermal performance of an IC chip is especially important when usinghigh power IC chips that require a high capacity heat sink.

FIGS. 8A and 8B illustrate the current practice of locating solder jointconnections on the bottom of a PBGA substrate for coupling the PBGAsubstrate to a PWB. FIG. 8A is a bottom view of the PBGA substrate 20.The substrate 20 typically has two distinct regions, the first region 32is underneath the shadow of an IC chip 10, and the second region 30 isoutside the IC chip shadow. The "shadow" of the IC chip comprises thearea underneath the chip in other structures like the laminate.

FIG. 8B is a cross section of the flip chip package that illustrates thetypical practice of locating the solder joint connections 31 between thePBGA substrate 20 and the PWB 11, which is to locate the solder balls 31in the region outside of the chip shadow 30. With current flip chippackaging techniques, there is a tendency for the IC chip to bulge up orwarp as previously discussed. This tendency for warpage places a mixtureof shear stresses and bending stresses on the solder ballinterconnections 31.

FIGS. 9A and 9B illustrate an embodiment of the present invention thatreduces warpage or the bulging of the IC chip by using sacrificial orredundant solder joint connections by locating them in the shadow regionof the IC chip. FIG. 9A is a bottom view of the PBGA substrate 22 withthe shadow region 32 of the IC chip and the region 30 outside of theshadow of the IC chip. As in current practice, the solder jointconnections 31 carry the signals including power and ground from the PWBthrough the PBGA laminate to the IC chip. This embodiment of the presentinvention, however, uses additional or redundant voltage and groundsolder joint connections 33 located within the shadow region 32 of theIC chip.

FIG. 9B is a cross section view of FIG. 9A that better illustrates theshadow region 32 and region 30 outside of the shadow region as used inthis embodiment of the present invention. The use of the additionalsolder ball connections 33 allows the mixture of shear stresses andbending stresses, which result from the bonding and encapsulation of theIC chip 10 to the laminate 20, to be spread over a larger number ofsolder joint connections (that now includes 31 and 33) to the PWB 11.The additional solder joint connections as a whole help reduce the shearand bending stress on each individual solder joint connection. Some ofthese "sacrificial" solder joint connections near the center of theshadow region 32 may eventually break, shear, or crack. This is not aconcern, however, since the solder joint connections 32 within theshadow region are best utilized for "redundant" interconnections forvoltage and ground between the PWB and the IC chip.

The present invention is a collection of techniques that overcome theabove disadvantages and improve the overall reliability of the flip chippackaging technique. One may practice the different techniques of thepresent invention separately or in combination with one another. Thepresent invention reduces the probability of an IC chip crack due toexcessive stresses such as from warpage. Additionally, the presentinvention reduces the probability of an interfacial delamination betweenthe IC chip and the encapsulant and or between the encapsulant and thesubstrate. And, the present invention also improves the reliability ofthe flip chip packaging technique in its daily uses (as part of aproduct) under repetitive thermal cycles of normal use.

The present invention discloses one embodiment that relocates theneutral planes in the PBGA laminate. This embodiment of the presentinvention reduces the compression strain on the device side of the ICchip and reduces the tensile strain on the back side of the IC chip thatoccurs when the encapsulated flip chip package assembly cools from itswiring board assembly temperature to room temperature.

Another embodiment of the present invention controls the stiffness ofindividual metal layers within the PBGA laminate with a reduction ofstiffness from the bottom side to the top side of the laminate. Thisembodiment reduces warpage and the shear strain on the IC chip solderjoints.

Another embodiment of the present invention uses redundant solder jointconnections between the flip chip package and the printed wiring board.These additional solder joint connections help reduce the bending of theIC chip and help improve the placement of any heat sink on the back sideof the IC chip. Reducing the bending of the IC chip also helps improveits thermal performance with a heatsink.

We claim:
 1. A laminate comprising:a plurality of metal layers varyingin flexibility; a plurality of dielectric layers coupled to said metallayers such that said metal and dielectric layers are stackedalternatively one above another to form a stack; and, at least one viahole disposed and extending through said plurality of said metal anddielectric layers, wherein said via hole increases in size as said viahole extends through said stack from one side of said stack to theother, such that said metal layers become more flexible as the size ofsaid via hole increases.
 2. The laminate of claim 1 wherein said viahole is circular having a diameter and said diameter increases in sizeas said via hole extends through said stack from one side of said stackto the other, such that said metal layers become more flexible as saiddiameter of said via hole increases in size.
 3. The laminate of claim 1wherein said laminate is for use in a plastic ball grid array substrate.4. A flip chip package, comprising:a plurality of metal layers varyingin flexibility; a plurality dielectric layers coupled to said metallayers such that said metal and dielectric layers are stackedalternatively one above another to form a stack; and, at least one viahole disposed and extending through said plurality of said metal anddielectric layers, wherein said via hole increases in size as it extendsthrough said stack from one side of said stack to the other, such thatsaid metal layers become more flexible as the size of its hole openingincreases in size.
 5. The flip chip package of claim 4 where theflexibility of each of said metal layers varies according to thediameter of said via hole in said metal layer.